The present invention relates to ceramic honeycomb bodies and process for the manufacture thereof, and in particular to ceramic honeycomb bodies having very thin walls and high surface areas for use for example, in automotive catalytic converters.
Ceramic honeycomb bodies are well known in the automotive industry for their application in the purification of automobile exhaust gas purification. Such structures are at the basis of the catalytic converter system, acting as support substrates to catalysts.
More and more automobile manufacturers are now demanding improved conversion efficiency and faster light-off to meet new engine designs and stricter regulations. These requirements having recently translated into honeycomb structures with very thin walls (xe2x89xa60.10 mm) and extremely high geometric surface areas (600-900 cells/in2). However, such design features although beneficial in terms of emissions performance, are not without tradeoffs.
Specifically, very thin wall/high surface area honeycomb structures have low isostatic strength. As a result during the canning process, a required step prior to use in a catalytic converter system, the honeycomb structure is prone to suffer cracking and chipping which has been observed to ultimately lead to catastrophic failure.
Attempts have been made to address this problem. However, approaches suggested have created additional disadvantages, such as a decrease in the thermal shock resistance and an increase in the pressure drop, both of which are critical to good product performance.
Therefore, there remains a need for a ceramic honeycomb body which provides a optimal combination of increased isostatic strength, high thermal shock resistance, and minimum increase in pressure drop for use in automotive exhaust gas purification.
The present invention relates to honeycombs of improved configurations that offer a significant increase in isostatic strength to resist cracking and chipping damage during handling and canning. At the same time, the bodies of the invention retain favorable thermal shock resistance, and promise resistance to an increase in pressure drop at least equivalent to currently available very thin wall/high surface area honeycomb structures absent the design features of the inventive honeycombs. For purposes of the present invention xe2x80x9ccurrently available very thin wall/high surface area honeycomb structuresxe2x80x9d shall be referred to in the description of the invention as xe2x80x9cstandard thin wall honeycombsxe2x80x9d.
In particular, the invention provides a honeycomb including a plurality of parallel cells defined by intersecting internal walls, and arranged in horizontal and vertical rows at and between opposing ends of the honeycomb body, an outer peripheral wall surrounding the cells, and further being interconnected to the internal walls, wherein the cells are divided into a first region including a portion of the cells adjacent the outer peripheral wall, and a second region including the remaining cells, wherein the cells in the first region have a wall thickness that is continuously increased along an axis extending to the outer peripheral wall, wherein fillets are formed at least at intersections between the interior walls in the first region of cells, fillets having radii which are continuously increased along an axis extending to the outer peripheral wall. Therefore, in the present invention both the wall thickness and the fillet radius are gradually increased towards the outer periphery of the honeycomb to increase isostatic strength while maintaining a high thermal shock resistance.
An extrusion die for fabricating the honeycomb article according to the present invention is provided with a die body including an inlet face, a discharge face opposite the inlet face, a plurality of feed holes extending from the inlet face into the die body, an intersecting array of discharge slots extending into the die body from the discharge face to connect the feed holes at feed hole/slot intersections within the die, the slots being formed by a plurality of pins, wherein a plurality of discharge slots near an outer periphery of the die have a width that is continuously increased along an axis extending to the outer periphery of the die, wherein a plurality of pins near the outer periphery of the die have rounded corners.
In the method of making the extrusion die there is first provided a die body incorporating an inlet face, a discharge face opposite the inlet face, a plurality of feed holes extending from the inlet face into the body, and an intersecting array of discharge slots extending into the body from the discharge face to connect with the feed holes at feed hole intersection within the die, the intersecting array of discharge slots being formed by side surface of an array of pins. Next, there is provided an electrical discharge electrode which includes a plurality of openings formed by a network of intersecting webs having a continuously increasing width in an axial direction to an outer periphery of the electrode, and rounded corners.
Then, the electrical discharge electrode is brought into contact with a plurality of pins on the discharge face of the die body to effect a reduction of pins located in a region adjacent an outer periphery of the die, the reduction being symmetrically on all side surfaces of the pins, concurrently with rounding of the pin corners thereof. As such the resulting die comprises an array of pins wherein a plurality thereof have rounded corners and form discharge slots having a width that is continuously increased along an axis extending to the outer periphery of the die, with the remaining pins being un-machined by the electrical discharge electrode.